1. Filed of Industrial Utilization
The present invention relates to a three-band antenna for use in automobiles, which is capable of receiving AM and FM bands and is also capable of transmitting and receiving MT band.
2. Prior Art
Conventionally, antennas for AM and FM broadcast bands (hereafter called "AM/FM antennas") and antennas for transmitting and receiving an MT band (communications band used for automobile telephones, and hereafter called "MT antennas") are installed separately. However, because of problems in terms of external appearance and difficulty in installation, etc., the development of single three-band antennas has been demanded, and such antennas have now reached the stage of practical use. Examples of this type of antenna are disclosed in Japanese Patent Application Laid-Open (Kokai) Nos. 62-173801 and 62-179202.
The antennas disclosed in these Laid-Open publications are simple rod-form antennas and in a form of telescopic antennas (which are free to extend and retract). In the antennas of this structure, it is possible to use a single antenna for the three bands; however, the antenna involves such problems as follows:
Specifically, in the case of an automobile antenna for AM/FM bands, as shown in FIG. 3, an antenna element 1 is inclined with respect to the top surface of vehicle 2 in order to avoid wind pressure and to avoid contact with obstacles when the vehicle is parked in a garage, etc. Such an inclined antenna is also for the purpose of increasing the reception sensitivity in the FM and AM bands. Since FM waves are horizontally polarized waves or circularly polarized waves, the reception sensitivity improveds by the antenna inclined as described above.
Meanwhile, MT antennas must be non-directional within the horizontal plane. Accordingly, such antennas are installed on the top surface of the vehicle body in a perpendicular position so that the transmission and reception are performed with vertically polarized waves.
As described above, it is desirable that the AM/FM antennas be installed to the top surface of the vehicle in an inclined position and the MT antennas installed in a perpendicular position. Thus, these requirements cannot be satisfied as long as a single rod-form antenna of the type described above is used.
For both AM/FM antennas and MT antennas, it is desirable, from the standpoint of transmission and reception efficiency, that the antenna be installed on the roof of the automobile which is the highest part of automobile above the ground. In Europe, the antennas are commonly mounted on the roof of automobiles in view of design preference.
The object of the present invention is to provide a three-band antenna for use in automobiles which has a sufficiently high reception sensitivity in the AM and FM bands, allows good transmission and reception in the MT band, and has a high transmission and reception efficiency.